1. Field of the Invention
The present invention relates to a laminated band-pass filter in which a plurality of dielectric layers and electrode layers are laminated.
2. Description of the Related Art
A traditional high-frequency band-pass filter suited for reducing its size and cost is one in which a plurality of LC resonators is disposed in a laminated structure of dielectric layers and electrode layers. Examples of such a laminated band-pass filter are disclosed in Japanese Unexamined Patent Application Publication No. 4-6911, Japanese Unexamined Patent Application Publication No. 2000-201001, Japanese Unexamined Patent Application Publication No. 2003-198226, and International Publication No. WO 02/009225
The configuration of the laminated band-pass filter in Japanese Unexamined Patent Application Publication No. 4-6911 is described with reference to FIGS. 1A and 1B. FIG. 1A is a circuit diagram thereof, and FIG. 1B is a cross-sectional view thereof. In the filter, a plurality of LC parallel resonant circuits are inductively coupled (magnetically coupled). Inductors L1, L2, L3, . . . , and Ln and capacitors C1, C2, C3, . . . , and Cn form a plurality of parallel resonators, and the inductors of neighboring resonators are magnetically coupled.
As illustrated in FIG. 1B, a capacitor electrode pattern 12 and an inductor pattern 13 are formed by printing in each of a first layer 10-1, a second layer 10-2, and a third layer 10-3, and these layers form resonators. That is, a capacitor is formed between a ground electrode 11 and the capacitor electrode 12, and the inductor patterns 13 of two layers are electrically connected to each other via a blind through hole 14. A plurality of layers forming such resonators are laminated as a fourth layer 10-4 and its underlying layers, thus magnetically coupling neighboring inductors.
Japanese Unexamined Patent Application Publication No. 2000-201001 describes a laminated band-pass filter in which a plurality of LC resonators is disposed inside a laminated structure of dielectric layers and electrode layers. The plurality of LC resonators are made up of a plurality of capacitances formed by a plurality of capacitance forming electrodes and inductances included in the plurality of capacitance forming electrodes. The neighboring LC resonators of the LC resonators are disposed inside the laminate structure at different height positions in the thickness direction of the laminated structure and electromagnetically coupled. The arrangement in which the plurality of LC resonators are disposed in the laminated structure at different height positions in the thickness directions of the laminated structure can reduce the part size while a physical distance between the LC resonators that is necessary in the design of a band-pass filter is maintained.
The laminated band-pass filter in Japanese Unexamined Patent Application Publication No. 2003-198226 includes a filter element in which first and second filter lines each including a pair of parallel lines disposed in part of a wiring layer are opposed to different circuit layers and are electrically connected at one end and the pair of lines is folded through a dielectric layer.
The laminated band-pass filter in International Publication No. WO 02/009225 includes two strip lines forming a resonator and disposed in the same layer at a specified interval and thereby electromagnetically coupled.
For the laminated band-pass filter of Japanese Unexamined Patent Application Publication No. 4-6911, the inductor included in each of the LC parallel resonators is made up of a two-layer inductor pattern, so a problem arises in which magnetic coupling between the LC parallel resonators is large. The inductor made up of the two-layer inductor pattern causes a problem in which a decrease in the Q value of the inductor increases the insertion loss of the laminated band-pass filter. To solve the above problems, it is necessary to have a sufficiently long distance between the LC parallel resonators. This results in a problem in which the thickness of the laminated band-pass filter is large.
The laminated band-pass filter of Japanese Unexamined Patent Application Publication No. 2000-201001 uses self resonance of the capacitor. The capacitance component of each of the capacitor electrodes and the inductance component of the capacitor electrode form an LC resonator. Because of this, the resonator cannot have a desired inductance, and characteristics of low insertion loss of the laminated band-pass filter are unobtainable.
The laminated band-pass filter of each of Japanese Unexamined Patent Application Publication No. 2003-198226 and International Publication No. WO 02/009225 can achieve a small size and low insertion loss. However, if a multilayer resonator is used to achieve sharp attenuation characteristics from a passband to a range outside the passband, two-stage filters are laminated in the direction of the lamination and strip lines are coupled in the direction of the thickness. This results in a problem in which the multilayer filter has a large thickness.
For these traditional laminated band-pass filters, when an LC parallel resonator made up of a capacitor electrode and an inductor electrode is arranged in a laminated structure and neighboring inductor electrodes are inductively coupled, there is a problem in which a ripple (deviation) occurs in passband characteristics.